SPREAD SPECTRUM

PRINCIPLES

ECE2526 – MOBILE COMMUNICATION

Monday, March 9, 2020

CONCERNS IN COMMUNICATION SYSTEM

1. Most focus in communication system study is focused on efficient use of signal energy and bandwidth.

2. There are however circumstances where it is necessary for the communication system to:(a) resist external interference;

(b) operate with low spectral density

(c) provide Multiple Access without external control

(d) Make it difficult for unauthorized receivers to decode the signal.

3. Spread Spectrum is one of the mostly applied technique to achieve this objective.

WHEN IS A COMMUNICATION SYSTEM

CONSIDERED AS SPREAD SPECTRUM?

A system is considered to be spread

spectrum if is satisfies these two conditions:

a) The bandwidth of the transmitted signal is

much greater than the message bandwidth,

and

b) The transmitted bandwidth is determined by

some function that is independent of the

message

ADVANTAGES OF SPREAD SPECTRUM

Spread spectrum systems do not combat noise unlike systems like PCM and FM, they however have the following advantages:

a) Has anti-jam capability, especially narrow-band jamming

b) Provides interference rejection

c) Has multiple Access Capability

d) Protects against multipath interference

e) Has lower probability of intercept (or is use for covert operation)

f) Provides secure communication

CLASSIFICATION OF SPREAD SPECTRUM SYSTEMS

• Spread Spectrum system are classified according to

the modulation system used as:

a) Direct sequence (pseudo-noise) where data is

scrambled using user specific pseudo noise code

b) Frequency Hopping where the signal is spread

by changing the frequency over the transmitted

time.

c) Time hopping where data is divided into frames

with each frame being split further into time

intervals. Each data burst is then hopped over

frames by utilizing code sequences.

DIRECT SEQUENCE/PSEUDO NOISE TRANSMITTER

• In Direct Sequence(also called a Pseudo-noise), the

signal is generated by adding a binary message with the

output of a pseudo-noise generator as shown below:

Binary

AdderBalanced

Modulator

Carrier

𝑓𝑐

Pseudo-noise

Generator

Clock

Message

Output

Binary

AdderBalanced

Modulator

Carrier

𝜔𝑜

Pseudo-noise

Generator

Clock

m(t)

p(t)

m(t)p(t) y(t)

DIRECT SEQUENCE/PSEUDO NOISE RECEIVER

1.The receiver of the Pseudo noise signal performs

three distinct functions, i.e1. detect the presence of signal,

2. de-spread it and

3. demodulate the message.

2.The de-spreader multiplies the incoming signal with

a locally generated PN sequence.

3.When the PN sequence and the incoming signal are

aligned, the output from the de-spreader is the

original message which can be filtered and

demodulated to produce the original.

De-spreader Narrowband

FilterMessage

Demodulation

PN Code

Generator

Local Clock

Reconstruction

𝑠(𝑡) 𝑚(𝑡)

DIRECT SEQUENCE/PSEUDO NOISE RECEIVER (2)

Multiplies the

incoming signal

with a locally

generated PN

sequence.

Original

message

FREQUENCY HOPPING SPREAD SPECTRUM

1. Frequency-hopping spread spectrum (FHSS) is a method of

transmitting radio signals by rapidly switching a carrier among

many frequency channels.

2. A pseudorandom sequence known to both transmitter and receiver

is used by the frequency synthesizer.

TIME HOPPING SPREAD SPECTRUM

• A time hopping

system is a spread

spectrum system in

which the period or the

duty cycle of a pulsed

RF carrier are varied

in a pseudorandom

manner under the

control of a coded

sequence.

HYBRID DS/FH SPREAD SPECTRUM SYSTEM

Frequency

SynthesiserPN Code

Generator

Data

RF Oscillator

~

Hybrid DS/FH

Output

Baseband DS

Output

HISTORY OF SPREAD SPECTRUM

1941: First publications.

1949: C. Shannon and R. Pierce develop basic

ideas of CDMA.

1950s: First applications as military Anti-jam

applications

1980s: Investigations for cellular use 80s.

1993: IS-95 standard.

1995: First commercial introduction.

1997/1998: Applied as 3G technology – WCDMA

and UMTS

CDMA PRINCIPLE

Message signal

Pseudo Code

+

Each bit of sequence 1 is replaced

by the code sequence

Output

Rate of Change Known as bitrate

Rate of Change Known

as chiprate

CDMA SIGNAL SPECTRUM

Message Signal

Output Signal

Bandwidth = 2KHz

Bandwidth = 16KHz

The spectrum has spread from 2 Hz to 16 Hz, i.e by a factor of 8.

THE SPREADING FACTOR

1. Multiplication with the code sequence which is of a higher bit rate, results in a much wider spectrum.

2. The ratio of the code rate to the information bit rate is called the spreading factor of the spread spectrum system.

3. In CDMA (IS-95), the spreading factor is 64.

GENERSATIONS OF MOBILE DATA STANDARDS

High Speed Uplink

Packet Access

High Speed Downlink

Packet Access

SAFARICOM WCDMA

SAFARICOM HSDPA

CDMA

1. CDMA uses unique

spreading codes to spread

the baseband data before

transmission.

2. The signal is transmitted in

a channel (BTS to BSC).

3. The receiver then uses a

correlator to de-spread the

wanted signal

4. The output of the correlator

is passed through a narrow

bandpass filter.

5. Unwanted signals are not

de-spread and do not pass

through the filter.

BASIC CDMA COMMUNICATION SYSTEM

INTERLEAVER

• Interleaving is used to facilitate error correction when a burst

error affecting consecutive bits occurs.

• Interleaved signals can therefore be easily recovered when

fast fading of signals occurs over the air while non-

interleaved are not.

EVOLUTION OF CDMA/1

1. The CDMA history can be directly linked back to the 1940s when this form of transmission was first envisaged.

2. In the 1950s, it started to be used for covert military transmissions in view of the facts that the transmissions look like noise, it is difficult to decipher without the knowledge of the right codes, and furthermore it is difficult to jam.

3.With the revolution in cellular telecommunications that occurred in the 1980s a then little know company named Qualcomm working on CDMA as a basis for a cellular telecommunications multiple access scheme -CDMA - code division multiple access..

4. Qualcomm was joined by US network operators Nynex and Ameritech to develop the first experimental CDMA system.

5. With the support of the Cellular Telecommunications Industry Association (CTIA) and the Telecommunications Industry Association (TIA) a standards group was set up. This group then published the standard for the first CDMA system in the form of IS-95 in 1995.

6. This development lead on to the CDMA2000 series of standards.

7. Later, it became necessary to evolve the GSM standard so that it could carry data and provide significant improvements in terms of spectrum use efficiency.

8. Accordingly CDMA, in the form of Wideband CDMA (WCDMA) was adopted for the GSM standard

EVOLUTION OF CDMA/2